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Energy-Structure Correlation in Metalloporphyrins and the Control of Oxygen Binding by Hemoglobin
Proceedings of the National Academy of Sciences of the United States of America
Vol. 74, No. 5 (May, 1977), pp. 1789-1793
Published by: National Academy of Sciences
Stable URL: http://www.jstor.org/stable/67108
Page Count: 5
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The contribution of the porphyrin skeleton to the potential energy surface of metalloporphyrins is calculated by the semiempirical method of quantum mechanical extension of the consistent force field to π electron molecules. This calculation makes it possible to correlate the observed structure of metalloporphyrins with the strain energy of the porphyrin skeleton. It is found that the out-of-plane metal displacement in pentacoordinate heme systems is due to both the restricted size of the porphyrin hole and the ``1-3'' steric interaction between the axial ligand and the heme nitrogens. The main components of the active site of hemoglobin are simulated by a histidine-heme-oxygen system. The energy surface of this system provides a quantitative explanation for the control of ligand binding by hemoglobin. It is shown that the heme acts as a diaphragm, designed to provide simultaneous binding to the histidine and the sixth ligand under the steric requirements of the 1-3 interactions. The dependence of the hemoglobin potential surface on the distance between the proximal histidine and the heme plane is evaluated for the R and T states, using the calculated heme potential and the observed energy of heme-heme interaction.
Proceedings of the National Academy of Sciences of the United States of America © 1977 National Academy of Sciences